[boost] Proposal: Visitor

From: Mac Murrett (mmurrett_at_[hidden])
Date: 2002-07-02 23:45:37

I have been looking at the implementations of variant classes 
floating by here, and while I have been happy with aspects of them, 
I am not happy with the visitability of any of them.  I therefore 
set out to provide an ideal visitor base class, based on some 
previous work of mine: it should be templatized on a type list (no 
holy wars on what type of type list, please :) ), should provide an 
interface consisting of a visit function for each type in the type 
list (that takes a reference to the type), and should have a 
pure-virtual back-end.  The trick is to have the public interface 
be nonvirtual to avoid hidden virtual function problems.

The implementation (with an example) is available at 
http://groups.yahoo.com/group/boost/files/visitor.zip.  As it uses 
type lists, it requires MPL.  It has been verified on CW 7.2, CW 8, 
and GCC 3.1.  It *does not* work won GCC 2.95.2.

Comments appreciated,
Mac Murrett.